THE MECHANISM OF SEX-DETERMINATION 59 



sea-urchins. We may take one cross as typical. When 

 eggs of strongylocentrotus are fertilized with sperm of 

 sphaerechinus, it is found at the first division of the egg 

 that, while some of the chromosomes divide and pass 

 normally to the two poles, other chromosomes remain 

 in place, or become scattered irregularly between the 

 two poles, as shown in Fig. 33. When the division 





It 



^'""/iV- 



» 







v '/^' 





Fig. 33. — 1 and la, chromosomes in the normal first cleavage spindle of 

 Sphaerechinus ; 2, equatorial plates of two-cell stage of same ; 3-3a, hybrid, 

 Sphaerechinus by Strongylocentrotus, spindle at two-cell stage ; 4-4a, same 

 equatorial plates; 5-5o, hybrid, Strong, by Sphaer., cleavage spindle in telo- 

 phase ; 6, next stage of last ; 7, same, two-cell stage ; 8, same, later ; 9, same, 

 four-cell stage ; 10, same, equatorial plate in two-cell stage (12 chromosomes) ; 

 11, same, from later stage, 24 chromosomes. (After Baltzer.) 



is completed, some of these chromosomes are found 

 outside of the two main nuclei. They often appear 

 as irregular granules, and show signs of degeneration. 

 They are still present as definite masses after the next 

 division, but seem to take no further part in the de- 

 velopment. 



Baltzer has attempted to count the number of chro- 

 mosomes in the nuclei of these hybrid embryos. The 



